1. Field of the Invention
The present invention relates to articles made from metal-ceramic composite materials featuring a particular kind of fibrous reinforcement. In particular, the invention relates to metal-ceramic composite mirrors having low thermal expansion coefficient.
2. Discussion of Related Art
Light-weight mirrors and structures are required in many government as well as commercial systems, including space systems. There are some very specific property demands on the materials used for construction of these mirrors and structures. These include lightweight, high stiffness, low to zero coefficient of thermal expansion (CTE), high thermal conductivity, polishability and adequate strength and toughness. Traditionally, low or ultra low expansion glasses and glass ceramics have been used along with metallic coatings for mirror constructions. However, there are several disadvantages of glasses such as low specific stiffness, low toughness, etc. Recently, beryllium, silicon carbide, and carbon fiber/polymer composites have been proposed for this application. However, each of these has its own limitations such as toxicity, non-zero expansion (Be); shape making difficulty, non-zero expansion (SiC); and environmental instability and low thermal conductivity (Cf/polymer). The only way to achieve zero-CTE in a composite requires use of negative CTE reinforcement, such as ultra high modulus carbon fibers as a reinforcement.
It has been known for a long time to add fibrous reinforcement to metals to increase mechanical properties such as specific strength and specific stiffness. One of the early such reinforcements was carbon or graphite fiber, produced from polymer precursors. The resulting composite material offered double or triple the strength or stiffness compared to the bulk, unreinforced metal. Processing was difficult, however, as the metals either tended not to wet the carbon fibers, or reacted with the carbon. Considerable energy has been devoted to developing ways to preserve the chemical and physical integrity of the fibers while rendering them more chemically compatible with the metal matrix.
Carbon fibers can be manufactured with high degrees of anisotropy. The graphite form of carbon in particular features a hexagonal crystallographic structure, with the covalent bonds within the {001} planes being strong, and the bonds between the {001} planes consisting of weak van der Waals bonds. It is possible to preferentially align the crystallographic planes in a graphite fiber such that the {001} planes tend to be parallel to the graphite fiber axis. By increasing the relative amount of covalent bonds in the fiber axis direction, a fiber possessing high strength and high elastic modulus in the direction of the fiber axis is produced. An interesting and unusual phenomenon that accompanies the alignment of the high strength, high modulus direction is that this particular direction also possesses a negative CTE. Thus, instead of expanding upon heating like most materials, these fibers actually shrink in the axial direction. In the radial direction of such fibers, however, the strength and elastic moduli are relatively low and the CTE is positive and relatively high.
When a reinforcement material having a negative CTE is incorporated into a composite material whose matrix component has a positive CTE, the individual CTE's tend to offset or cancel one another, yielding a composite or overall CTE somewhere between the two values. Because of this counterbalancing or offset effect, it is theoretically possible to engineer a metal matrix composite material, such as a metal-ceramic composite material, to have a net overall CTE of zero. By incorporating parallel arrays of such fibers into a positive CTE isotropic matrix, a composite material having a high modulus and a zero or near-zero CTE in the axial direction of the fibers can be produced. In the direction transverse to the fiber axes, the modulus would be relatively low and the CTE would be relatively high. Because of the axial stiffness, the properties of the composites tend to be dominated by the axial properties.
The degree of anisotropy can be reduced by distributing the fiber orientations. One technique for accomplishing this is to arrange the fibers in parallel within a thin sheet or “ply”, and to place a number of such plies on top of one another such that fibers in one ply are skew with respect to fibers in an adjacent ply. With suitable arrangements of the plies it is possible to produce quasi-isotropic sheet materials. Quasi-isotropic lay-ups of thin plies of the composite can be achieved by orienting successive plies at 0°, +45°, −45° and 90°; or 0°, +60° and 60° with respect to the fiber axes. The distribution of the fiber directions, however, significantly reduces the CTE influence of the fibers (as will be illustrated later); thus, it becomes that much more difficult to produce composites that have zero or near-zero CTE's in the dominant plane of the composite.
U.S. Pat. No. 3,807,996 to Sara teaches a carbon fiber reinforced nickel matrix composite material. Sara discloses the use of high strength, high modulus carbon fibers, as well as various geometrical arrangements of the fibers, such as arrays (plates) of parallel fibers and cross-plies (laminates) of such arrays.
U.S. Pat. No. 4,083,719 to Arakawa discloses a carbon fiber reinforced copper composite featuring a low thermal expansion coefficient and no directional characteristic of the mechanical properties. The resulting composite bodies featured CTE as low as 4×10−6 cm/cm per degree K (hereinafter referred to as “parts per million per degree Kelvin” or ppm/K).
U.S. Pat. No. 4,157,409 to Levitt et al. discloses treating carbon fibers with molten NaK to permit wetting by molten aluminum, magnesium, copper, zinc, tin or lead matrix metals.
U.S. Pat. No. 5,834,115 to Weeks, Jr. et al. discloses protecting carbonaceous reinforcement materials such as fibers with molybdenum carbide and then infiltrating with a molten metal to produce a composite body. A woven fabric of coated graphite fibers reinforcing a copper matrix exhibited a CTE between about 4 and 7 ppm/K).
High modulus carbon fibers have also been incorporated into polymeric matrices. U.S. Pat. No. 5,330,807 to Williams discloses a composite laminated tubing intended for offshore oil extraction operations. There the problem was the need to transfer oil in a tube over appreciable distances and in which the tube may undergo considerable temperature excursions due to the elevated temperature of the extracted oil. To minimize the expansion of the tube length and thereby ameliorate the propensity for the tube to fail by buckling, the tubing is made of a plurality of layers of fibers fixed in a plastic matrix. The fibers may be graphite fibers, glass fibers, ceramic fibers or polymer fibers, but in any case the fibers have a sufficiently low CTE as to impart to the tubing an overall CTE of no more than about 1.1 ppm/K, and a Poisson's ratio near 0.5.
In many environments, however, polymer matrix composites cannot be used because of insufficient resistance to extremes of temperature, corrosion or radiation. Accordingly, some workers have used glass as the matrix material. Glass has several attractive properties for these types of materials, including fluidity or flowability, wettability to the fibers and the potential for relatively low CTE's. For a laser mirror application, for example, Stalcup et al. (U.S. Pat. No. 4,451,118) hot pressed a mixture of a low expansion borosilicate glass and alternating plies of high modulus graphite fibers. Some of the graphite fibers were arranged perpendicular to the reflecting surface so as to be better able to conduct heat away from the mirror surface. Still, cooling passages had to be placed into the mirror structure to permit circulation of a heat exchange fluid.
Similarly, in U.S. Pat. No. 4,791,076 Leggett et al. discloses a graphite fiber silica matrix composite composition having a near-zero overall CTE. In addition to silica, the matrix contains boron phosphate and beta-spodumene, and Leggett states that the composite CTE is tailorable between −1 and +1 ppm/K by varying the matrix composition. As a consequence of the low CTE, very little thermal distortion occurred in for example, a laser mirror application, particularly at low coolant flow rates. This glass matrix composite material exhibited much less thermal distortion than did other laser mirror materials such as single crystal molybdenum or silicon. Although the cooling requirements were reduced, active cooling techniques involving heat transfer media flowing through channels in the mirror still were required.
As mentioned above, glass matrix composites have been used in environments where low expansion polymer composites would be insufficiently durable. Many of these applications, however, require high thermal conductivity, and most glasses are deficient in this area. Thus, composites workers have attempted to address the thermal conductivity problem by relying on the carbon fibers to carry this responsibility, the carbon fibers possessing relatively high thermal conductivity in the fiber axis direction. Another problem with glass matrix composites, though, is that they tend to be brittle. In many applications in which such composites are subjected to accelerations and stresses, such as with semiconductor fabrication equipment, it would be preferable to have a tougher, more impact resistant material.
A number of metals such as aluminum and magnesium are intrinsically highly thermally conductive and tough, and possessing low specific gravity and sufficient durability in harsh environments as to make them candidates for aerospace or precision equipment applications. Unfortunately, these metals suffer from having relatively high CTE's—typically around 20 ppm/K or higher. There seem to be no successes or even proposals to make composites using these high modulus carbon fibers as the reinforcement of a light metallic matrix for the express purpose of producing very low CTE metal matrix composites. The lowest CTE achieved for such MMC's appears to be the 4 ppm/K of U.S. Pat. No. 4,083,719, which represents work done years ago. While quite low in comparison to unreinforced metals, there are applications, such as in optical systems that undergo temperature fluctuations, where even lower CTE's would be desirable.
The CTE of the composite body is also influenced by the elastic modulus of the individual composite constituents. More specifically, Equation 1 shows the mathematical relationship among CTE, elastic modulus, and volume fraction.
            Equation      ⁢                          ⁢      1        :                  ⁢          α      11        =                    V        ·                  E                      11            ⁢            f                          ·                  α                      1            ⁢            f                              +                        (                      1            -            V                    )                ⁢                              E            m                    ·                      α            m                                              V        ·                  E                      11            ⁢            f                              +                        (                      1            -            V                    )                ·                  E          m                    where:
V is the volume fraction of fibers;
Em is the elastic modulus of the matrix;
E11f is the elastic modulus of the fibers in the axial direction;
α1f is the CTE of the fibers in the axial direction;
αm is the CTE of the matrix; and
α11 is the CTE of the composite body in the fiber axial direction.
Obviously, the larger the contributing CTE's of reinforcement and matrix, the larger the overall CTE of the composite. As Equation 1 also demonstrates, however, the CTE of the composite is related to the elastic moduli of the matrix and reinforcement phases. Moreover, this equation shows that reducing the modulus of the matrix relative to the modulus of the reinforcement reduces the CTE contribution of the matrix to the overall composite CTE, therefore causing the composite CTE to trend toward the CTE of the reinforcement. For reinforcement materials possessing negative CTE such as certain high modulus carbon fibers (at least in the axial direction), the averaging of the matrix and reinforcement CTE's to yield the CTE of the composite tends to produce a CTE “cancellation”. Thus, it is at least theoretically possible with a proper balancing of CTE's to engineer the overall CTE of the composite to be zero.
One must bear in mind, though, that the situation described immediately above represents the case in which the fibers are aligned. Such a composite would be highly anisotropic, and the composite CTE would be zero or near-zero only in the direction parallel to the fiber axes. In other directions, the overall CTE of the composite body would be non-zero and positive.
Over the years, there has been a significant amount of work directed to reinforcing SiC composites, e.g., reaction-bonded SiC composites, with carbon fibers. See, for example, U.S. Pat. Nos. 4,118,894; 4,944,904 and 6,248,269. None of these patent documents expressly state low CTE as an objective; however, because the matrices of these composite materials consist of low CTE substances, i.e., SiC, typically interconnected, and typically also some residual, unreacted Si, also typically interconnected, the resulting composite bodies are expected to inherently feature a low CTE, e.g., on the order of about 2.6–2.8 ppm/K for Si/SiC composites. For example, U.S. Pat. No. 6,248,269 to Dietrich et al. discloses a reaction-bonded SiC composite suitable for braking applications, e.g., disk and pad, for motor vehicles, consisting of carbon fibers arranged isotropically and embedded in a matrix of 40–50 volume percent SiC and not more than 15 volume percent Si. The carbon fibers are protected from chemical reaction with the Si infiltrant by infiltrating a pitch resin into the mass of fibers prior to the Si infiltration step.
U.S. Pat. No. 4,944,904 to Singh et al. discloses a similar composite material system intended mostly for high temperature, aerospace applications, such as a turbine engine component, but also mentioning applications such as wear parts and acoustic parts. The matrix comprises at least 5 volume percent SiC but preferably at least 45 percent, and 1–30 volume percent Si but preferably 1–2 percent. The fibers may be carbon or SiC, but are not disclosed as being arranged isotropically or quasi-isotropically. The fibers similarly are protected from attack by the molten silicon, but here, the protective coating consists of boron nitride plus an overcoat of a silicon-wettable material such as carbon or metal carbides such as SiC. The BN also provides a debond layer so that the fibers can move relative to the matrix under mechanical loading, thereby providing a toughening aspect to the resulting composite body. Again, low CTE does not appear to be an express objective of either of these reaction-bonded SiC composite patents; however, the formed composites are expected to inherently possess a low CTE, as most of the constituents are low CTE materials.
Ealey et al. applied RBSC technology to the problem of fabricating a mirror requiring low CTE, high thermal conductivity, high elastic modulus and low density (“CERAFORM SiC: roadmap to 2 meters and 2 Kg/m2 areal density”, Advanced Materials for Optics and Precision Structures, SPIE Critical Reviews, Vol. CR67, pp. 53–70, ISBN 0-8194-2598-2, September 1997). Normally, it is difficult to obtain a smooth polish of a reaction-bonded SiC surface because the silicon phase tends to be removed at a much faster rate than does the SiC phase; however, Ealey at al. claim to have overcome this problem through careful control of the polishing conditions, particularly of the pH of the polishing slurry. They report a CTE for their CERAFORM SiC of about 2.44 ppm/K. Although such a CTE generally is considered to be low, there are applications that desire still lower CTE, or even zero CTE, if possible. Further, a key requirement of reaction-bonded SiC mirrors is polishability. Even if the SiC and Si phases can be polished at the same rate, they report that the polish rate is only ⅓ to ½ that of glass. Further, they have experimented with polishing amorphous Si coatings applied to CERAFORM SiC by a vapor deposition process.